Inside, we encounter the surface of the object that collapsed to make the black hole in the first place, and because we cannot tunnel inside this body, all we see is a solid surface. If the black hole was produced by a 'vacuum' phenomenon so that it required no mass to create it, the interior is hollow and has a pretty exotic geometry where time and space switch their roles, and in some cases, time travel may be possible. But these are not 'physical' black holes...only mathematical constructs not produced by any physical process we know about.

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